WiMAX, WiBro Measurements (Options K92/K93) R&S FSL
1300.2519.12 2.146 E-11
Fig. 2-90 Signal processing of the IEEE 802.16 OFDM measurement application
In the lower part of the figure the subsequent digital signal processing is shown. In the first block the
packet search
is performed. This block detects the Short Preamble (SP) and recovers the timing. The
coarse timing
is detected first. This search is implemented in the time domain. The algorithm is based
on cyclic repetition within the SP after
1282 ==
FFT
NN samples. Note this cyclic repetition occurs also
in the Long Preamble (LP). Numerous treatises exist on this subject, e.g. [1]–[3]. Furthermore a coarse
estimate
coarse
f
ˆ
5
of the Rx–Tx frequency offset f is derived from the metric in [6]. This can easily be
understood because the phase of )()(
*
Nirir + is determined by the mod 2 frequency offset. As the
frequency deviation
f can exceed several bins (distance between neighbor sub–carriers) the SP is
further used to solve this n2 [offset over several bins] ambiguities.
After the coarse timing calculation the time estimate is improved by the fine timing calculation
. This is
achieved by first estimating the coarse frequency response
)S(
ˆ
P
k
H , with ]100,100[=k denoting the
channel index of the occupied sub–carriers. First the FFT of the SP is calculated. After the FFT
calculation the known symbol information of the SP sub–carriers is removed by dividing by the symbols.
The result is a coarse estimate
k
H
ˆ
of the channel transfer function. In the next step the complex
channel impulse response is computed by an IFFT. Next the energy of the windowed impulse response
(the window size is equal to the guard period) is calculated for every trial time. Afterwards the trail time
of the maximum energy is detected. This trial time is used to adjust the timing.
Now the position of the SP is known and the starting point of the useful part of the first payload symbol
can be derived. In the next block this calculated time instant is used to position the payload window
.
Only the payload part is windowed. This is sufficient because the payload is the only subject of the
subsequent measurements.
In the next block the windowed sequence is compensated
by the coarse frequency estimate
coarse
f
ˆ
.
This is necessary because otherwise inter channel interference (ICI) would occur in the frequency
domain.
The transition to the frequency domain is achieved by an FFT
of length 256. The FFT is performed
symbol–wise for every of the
symbolsnof _ symbols of the payload. The calculated FFTs are described
by
kl
r
,
with
5 In this paper the hat generally describes an estimate. Example: x
is the estimate of x.
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